1. Field of the Invention
The present invention relates to a novel method for discriminating between prostate carcinoma and benign prostatic hyperplasia. More specifically, the present invention relates to a novel method for discriminating between prostate carcinoma and benign prostatic hyperplasia based on the difference in glycan structure of prostate specific antigen (PSA).
2. Description of the Related Art
Prostate carcinoma (hereinafter, referred to as “PC”) is one of the major causes of death in males. Prostate specific antigen (hereinafter, referred to as “PSA”) has been recognized as the most important tumor marker for PC (for example, refer to Stamey et al., N. Engl. J. Med., 317, 909-916 (1987)). PSA is a glycoprotein having a molecular weight of approximately 30 kDa consisting of a protein portion consisting of 237 amino acid residues having a molecular weight of approximately 26 kDa, and a glycan portion linked to the amino acid residues (such as Asn45) of the protein portion, or a derivative or analog thereof. The glycan portion represents approximately 8% of the molecular weight of PSA. Many documents have already described the usability of serum PSA test for the initial diagnosis of PC. However, there is a region between males affected with benign prostatic hyperplasia (hereinafter, referred to as “BPH”) and males affected with PC, which is known as a gray zone, where it cannot discriminate between PC and BPH (for example, refer to Catalona et al., j. Am. Med. Assoc, 279, 1542-1547 (1998)). To solve the issue, several attempts have been made (such as discrimination based on PSA density, PSA gradient (annual increase rate) and the ratio of free PSA/total PSA). However, there is a significant overlap between the two lesions.
Recently, it has been reported that, in a study using the serial lectin affinity chromatography method using concanavalin A, phytohaemagglutinin E4 (PHA-E4) and PHA-L4, the structures of the glycans linked to asparagine (N) of PSA differ between the PC tissue and the BPH tissue (refer to Sumi et al., J. Chromatogr. B, 727, 9-14 (1994)). The report describes that the N-linked glycan in PSA is altered during the process of oncogenesis in the human prostate, and the N-linked glycan in PSA may serve as a diagnostic tool for PC.
As methods for detecting the PSA structure, several immunological approaches, using binding molecules which bind to a PSA glycan have been proposed. For example, a method for discriminating between PC and BPH has been reported, wherein the method brings PSA into contact with lectin, and measures PSA classified based on the affinity of PSA glycans to lectins (refer to Japanese Patent Laid-Open No. 2002-55108). In the report, it is described that the difference in the affinity of PSA glycans to lectin (Maackia amurensis lectin etc.) is based on the conformation of sialic acid at the glycan terminal. However, the specific structure of PSA glycan in subjects suffering from PC or BPH has not been determined. Moreover, difference in fucose modification to PSA glycan is yet to be found between PC and BPH.
Moreover, there is reported a method for detecting PC based on whether at least a triantennary glycan is present in PSA (refer to Japanese Patent Application Laid-open No. 2000-514919). In the method, a binding molecule which binds to at least triantennary glycans, but not to monoantennary and biantennary glycans is used. The binding molecules which can be used include lectin (such as PHA-L) and antibodies.
As another approach, several reports have been made focusing on fucose in a PSA glycan. For example, a method has been proposed for measuring the fucose content in a PSA glycan, and diagnosing PC when the content is abnormally increased (refer to GB patent application publication No. 2361060).
Alternatively, a method has been proposed for evaluating the clinical condition of a subject by determining the glycan profile of the glycans in a target glycoprotein including PSA (refer to Japanese Patent Application Laid-open No. 2006-515927). The document describes that, in an analysis using the MALDI-MS method, the PSA glycan in subjects suffering from PC has a different fucosylated structure from that of the normal PSA glycan. However, it does not describe the specific structure of PSA glycan in the subjects-suffering from PC.
Moreover, there has been made a report focusing on the content of fucose-bound glycan and fucose-unbound glycan in PSA (refer to Tabares et al., Glycobiology, 16(2), 132-145 (2006)). In the report, they analyzed the structures of PSA glycans only in one subject having a significantly high serum PSA concentration (1.8 μg/mL) by HPLC, and it describes that the ratio of the content of fucose-unbound glycan to fucose-bound glycan was 5.25 (84/16). On the other hand, there has been reported an analysis of glycopeptides prepared from serum PSA of two subjects suffering from PC having a further high serum PSA concentration (10 μg/mL or more) by MALDI-QIT-TOF MS (refer to Tajiri et al., Glycobiology Advance Access published Oct. 23, 2007 http://glycob.oxfordjournals.org/cgi/reprint/cwm117v1). The study discloses that the content of fucose-bound glycan is 100% and 64% respectively, and that the fucose-bound glycans are more abundant compared with the fucose-unbound glycans.
However, the structures of PSA glycans in subjects suffering from BPH is yet to be revealed, and discrimination between BPH and PC based on the specific structures of PSA glycans has not been performed.